You invented $10,000 in two accounts paying 8% and 9% annual interst, resepectively. IF the toatal interest earned for the year was $840, how much was invested at each rate?

Is that simple interest and the interest applied per annum?

If so, recall your interest formula:

I = PRT

Let x be the first sum and y the second.

I_1 = x(0.08)(1)

And

I_2 = y(0.09)(1)

And

I_1 + I_2 = 840

And

x + y = 10000

can you give that a try? :)